This invention relates to apparatus and methods for controlling cabin noise and vibration in propeller driven aircraft.
As described in U.S. Pat. No. 5,148,402 to Magliozzi et al., the asynchronous operation of propellers on multi-engine aircraft creates cabin noise and vibration. Each propeller creates airflow disturbances and beats as its blades turn through the air rushing by. Also, rotor imbalance acts on the propeller rotor shaft and is transmitted to and excites the aircraft structure.
On short trips, the acoustic noise and vibration may be bearable to cabin passengers. But on longer trips, the constant hum and vibration begins to wear on passengers and crew.
Conventional methods have been employed to try to cancel out the noise and vibration. Some are strictly electrical devices. They use microphones to sense cabin conditions, and send negative-phase electrical responses back to the cabin through strategically placed loudspeakers. These responses, though not instantaneous, tend to dampen the noise somewhat. They do little, however, to alleviate vibration.
Other conventional systems try to solve both problems. They use synchrophasing, in which the phase angle relationship between the rotating propellers is kept constant during a flight mode (e.g., liftoff or cruise). Each angle is pre-selected such that it promotes cancellation among the sources of noise and vibration. This requires a stable synchrophasing system of relatively good accuracy. Also, the phase angle required depends upon both the specific installation and the actual operating condition (particularly the propeller speeds). These prior systems typically employ synchrophasers which preset the possible propeller phase angles based upon testing of the development aircraft operating at selected nominal conditions. This approach does not generally provide the optimum phase angle to reduce cabin noise and vibration, since operating conditions and propeller speeds differ during various flights. A more flight condition-responsive synchrophaser is therefore needed.
Because of the cancellation/reinforcement functions of prior synchrophasers, it is also possible (for a chosen phase angle) that strong cancellation (and therefore low noise levels) can occur at one location in a cabin, while reinforcement (and therefore high noise levels) can occur at some other location in the cabin. It would therefore be desirable if a synchrophaser could also select the phase angles to balance the cabin noise so that there does not simultaneously exist any low noise locations and high noise locations.
Accordingly, it is a principal object of the present invention to provide an "adaptive synchrophaser" that overcomes the problems of the prior art.
It is another general object to provide a synchrophaser that readily adapts to ongoing flight conditions to properly adjust phase angle differences between propellers to cancel the sources of noise and vibration.
It is a more specific object to provide an "adaptive synchrophaser" that periodically samples the cabin noise and vibration environment, performs the desired analyses, and provides optimum phase angles for the actual configuration and operating conditions.
It is another specific object to provide an "adaptive synchrophaser" that can be incorporated into a synchrophaser or which could be a separate box that would work with an existing synchrophaser.